Analyzing Accuracy of a Battery Fuel Gauge System

Size: px

Start display at page:

Download "Analyzing Accuracy of a Battery Fuel Gauge System"

Ashley Baldwin
5 years ago
Views:

1 Analyzing Accuracy of a Battery Fuel Gauge System BMS Applications Onyx Ahiakwo & Jared Casey 1 1

2 Outline 1. Defining Accuracy 1. Measurement Accuracy 2. Gauging Accuracy 2. Common Terminology 3. How to Compute Accuracy 4. Test Cases 1. Common Loads 2. Variable Loads 3. Low Temperature 5. Questions 2 2

3 Defining Accuracy Measurement Accuracy v. Gauging Accuracy 3

4 Defining Accuracy A fuel gauge algorithm needs to have data from the battery. A gauge gathers data from the battery through various measurements. Gauge measurements include Battery voltage Current flowing into and out of the battery Battery temperature Measurement accuracy is dependent upon the gauge s hardware and is independent of gauging accuracy Gauging accuracy is dependent upon the robustness of the gauging algorithm and the gauge s measurement accuracy. Poor measurement accuracy can lead to poor gauging accuracy 4

5 Gauging Accuracy Gauging Accuracy Voltage Based Coulomb Counting Fuel Gauge w/ Impedance Track Algorithm Robustness 5

Gauging Accuracy Cell Voltage Measurement Measures cell voltage Advantage: Simple Not accurate over load conditions Coulomb Counting Measures and integrates

models that will vary with cell maker Can count the charge leaving the battery, but won t know remaining charge without complex models Models will become less

6 Gauging Accuracy Cell Voltage Measurement Measures cell voltage Advantage: Simple Not accurate over load conditions Coulomb Counting Measures and integrates current over time Affected by cell impedance Affected by cell self discharge Standby current Cell Aging Must have full to empty learning cycles Must develop cell models that will vary with cell maker Can count the charge leaving the battery, but won t know remaining charge without complex models Models will become less accurate with age Impedance Track Directly measures effect of discharge rate, temp, age and other factors by learning cell impedance No host algorithms or calculations 6

7 Measurement Accuracy Voltage Accurate voltage measurements are critical for Initialization of relaxed cell Updates during self-discharge of cell Correction for coulomb counting error TI fuel gauges use a 15 bit ADC to provide accurate voltage measurements Current Accurate coulomb counting is critical to Capture low sleep currents Capture short load spikes Capture proper passed charge TI fuel gauges have a dedicated 15 bit integrating ADC (i.e. coulomb counting hardware) Temperature Accurate temperature measurements are critical for Proper compensation of resistance Proper compensation of predicted runtime TI fuel gauges provide flexible temperature measurement options External measurements with cell s thermistor or board hotspot Host can write the temperature to the fuel gauge Internal IC temperature measurements 7

8 Common Terminology 8

9 Common Terminology Chemical Capacity (Qmax) Total chemical capacity of the battery, mah Remaining Capacity (RM) Useable capacity of the battery from current state to empty state, mah Full Charge Capacity (FCC) Amount of charge passed from a fully charged state until termination voltage, mah Relative State of Charge (RSOC) : 0% -100% RSOC=RM/FCC Depth of Discharge (DOD): 0% - 100% 0% is charged to the brim, 100% is completely empty of energy Does not depend on load or temperature or system characteristics Passed Charge (dq or passedq) Integration of current with respect to time. 9

10 Common Terminology-Cont d Coulomb A unit of electric charge. It is the charge carried by a one ampere current in one second Terminate Voltage (EDV) Voltage at which the system can no longer operate; target for 0% SOC Reserve Capacity Capacity that is left after reaching 0% SOC 10

11 How to Compute Accuracy 11

Memory (optional) Open Data Memory plug-in Click Auto Export Fully charge at room temp.

12 Conduct a Test Start with a properly configured gauge Ensure gauge has been calibrated For IT gauge, this means a properly created golden image Start logging: Log DataRAM registers Open Registers plug-in Click Start Log Log interval: 1 to 10 seconds Export Data Memory (optional) Open Data Memory plug-in Click Auto Export Fully charge at room temp. **Logging interval set in Window -> Preferences -> Registers Discharge to terminate voltage using desired load. **Export interval set in Window -> Preferences -> Data Memory 12

13 Testing Made Easy with the Gauge Development Kit 13

14 Simple inspection Log gauge data every 1-10 seconds Import to spreadsheet and plot voltage, SOC, current, etc. Inspect Does it report 0% at or near your true terminate voltage? If not, by how much capacity and/or run-time was it off? Does it look smooth or have large jumps? Very subjective and difficult to gauge relative error magnitude 14

15 Steps for Computing Accuracy 1. Start with gauge log 2. Calculate a new column: Calculated dq = rolling sum of current_reading*time_since_last_log_point Start from full condition and go to empty condition (EDV) 3. Calculate FCC_true = Integrated capacity from fully charged state down to termination voltage Simple integration: capacity = constant current * total discharge time from full to terminate voltage More accurate integration: calculate each log point Current*Tdelta and keep a rolling sum. 4. Calculate a new column: Calculated_RM = FCC_true calculated_dq True remaining capacity at any point (mah) 5. Calculate a new column: Calculated_SOC = Calculated_RM / FCC_true * 100 True relative state of charge (%) 6. Calculate a new column: SOC_error = (Calculated_SOC SOC_gauge) SOC_gauge is State of Charge in IT gauge log files 7. Plot voltage, RSOC_true, RSOC_gauge, RSOC_error 8. Summarize in table showing 1. % error at the point when termination voltage is reached 2. Max % error 15

Steps for Computing Accuracy Start with Gauge log Create new column for calculated passed charge (dq) Calculated_dQ = rolling sum of current_reading *

16 Steps for Computing Accuracy Start with Gauge log Create new column for calculated passed charge (dq) Calculated_dQ = rolling sum of current_reading * time_since_last_log_point Start from full and calculate until empty (or discharge stops) Excel formula: (ElapsedTime N+1 ElapsedTime N )* AvgCurrent N /3600+ Calculated_dQ N-1 16

17 Steps for Computing Accuracy Calculate FCC_true = Integrated capacity from fully charged state down to termination voltage Terminate Voltage Calculated FCC_true 17

18 Steps for Computing Accuracy Calculate a new column: Calculated_RM = FCC_true Calculated_dQ True remaining capacity at any point (mah) Excel formula: $FCC_true Calculated_dQ N 18

19 Steps for Computing Accuracy Calculate a new column: Calculated_SOC = Calculated_RM / FCC_true * 100 True state of charge at any point (%) Excel formula: Calculated_RM / $FCC_true *

20 Steps for Computing Accuracy Calculate a new column: SOC_error = SOC_true SOC_gauge Excel formula: Calculated_SOC N SOC_gauge N 20

21 Steps for Computing Accuracy SOC, Calculated SOC v. Time Voltage (mv) Time (s) Voltage SOC_gauge Calculated SOC SOC (%) 21

22 Steps for Computing Accuracy SOC Accuracy Voltage (mv) Error (%) Time (s) Voltage (mv) SOC Error (%)

23 Test Cases 23

24 bq27421 Standard C/5 Discharge I = -C/5 load Temp = 25 C TermV= 3000 mv Voltage SOC AvgCurrent Temperature Terminate Voltage reached 24

25 bq27421 Standard C/5 Discharge Max SOC error = 2.75% Voltage SOC SOC Error SOC EOD = 0.09% 25

26 bq27421 Variable Load I = -126 ma (C/10 load) I = -C/5 -> -C/1 -> -C/10; loop until TermV Temp = 25 C TermV = 3000 mv Voltage SOC AvgCurrent Temperature I = -252 ma (C/5 load) I = ma (C/1 load) Terminate Voltage reached 26

27 bq27421 Variable Load SOC EOD = -0.31% Voltage SOC SOC Error Max SOC error = 3.74% 27

28 bq27421 C/5 Discharge, Cold Temp I = -C/5 load Temp = 0 C TermV= 3000 mv Voltage SOC AvgCurrent Temperature Terminate Voltage reached Gauge reports 0% SOC ~10 minutes before Terminate Voltage is reached 28

29 bq27421 C/5 Discharge, Cold Temp Max SOC error = 5.41% Voltage SOC SOC Error SOC EOD = 4.28 % 29

30 bq27421 Variable Load with Reserve Capacity I = -126 ma (C/10 load) I = -C/5 -> -C/1 -> -C/10; loop until TermV Temp = 25 C TermV = 3000 mv Voltage SOC AvgCurrent Temperature dq I = -252 ma (C/5 load) ~10% capacity (126 mah) I = ma (C/1 load) Terminate Voltage reached 30

31 bq27421 Variable Load with Reserve Capacity Max SOC error = 3.30% Voltage SOC SOC Error SOC EOD = 2.31% 31

32 bq27742 Standard C/5 Discharge I = -C/5 load Temp = 25 C TermV= 3000 mv Voltage SOC AvgCurrent Temperature 32

bq27742 Standard C/5 Discharge SOC Error @ EOD = 0.

33 bq27742 Standard C/5 Discharge SOC EOD = 0.94% Voltage SOC SOC Error Max SOC error = -2.09% 33

34 bq27742 C Discharge I = -C load Temp = 25 C TermV= 3000 mv Voltage SOC AvgCurrent Temperature 34

35 bq27742 C Discharge Voltage SOC SOC Error SOC EOD = 0.84% Max SOC error = -2.72% 35

36 bq275xx Cold temp, C/5 Load I = -C/5 loop until TermV Temp = 0 C TermV = 3000 mv Voltage SOC AvgCurrent Temperature 36

37 bq275xx Cold temp, C/5 Load Voltage SOC SOC Error SOC EOD = 0.8% Max SOC error = -1.75% 37

38 bq275xx Constant Power, Cold Temperature Temp = 0 C TermV= 3250 mv P=3.65 W Voltage Temperature AvgCurrent SOC 38

39 bq275xx Constant Power, Cold Temperature SOC EOD = 0.01 % Voltage SOC SOC Error Max SOC error = -2.08% 39

40 Conclusions 40

41 Conclusion Gauging accuracy is dependent on the hardware measurement accuracy A more robust algorithm provides better accuracy. Impedance Tracking is the industry s most robust algorithm. To compute true accuracy, you need to know the actual capacity extracted from the battery for that particular test scenario. To compute true accuracy: Integrate current in order to compute actual capacity extracted from the battery (FCC true) Compute true remaining capacity (RM) by subtracting the integrated current from FCC true Compute true SOC by dividing RM by FCC True. Subtract gauge reported SOC from true SOC Use Impedance track for best gauging accuracy. 41

42 Questions?? 42

Maxim > Design Support > Technical Documents > Application Notes > Battery Management > APP 131

Maxim > Design Support > Technical Documents > Application Notes > Battery Management > APP 131 Keywords: battery fuel gauge, battery monitors, integrated circuits, ICs, coulomb counter, Li-Ion battery